This application is based upon and claims benefit of priority from the prior Japanese Patent Application No. 2000-276487, filed on Sep. 12, 2000; the entire contents of which are incorporated herein by reference.
1. Field of The Invention
The present invention relates generally to a liquid crystal display element.
2. Description of Related Art
Smectic liquid crystal materials having spontaneous polarization, such as ferroelectric liquid crystals and anti-ferroelectric liquid crystals, are expected as materials of next-generation liquid crystal display elements, since these materials have characteristics, such as rapid response and wide viewing angles, in a surface stabilized display mode. Particularly in recent years, it has been attempted to provide various active matrix liquid crystal displays (AMLCDS) for high quality moving picture. As materials which are suitable for this use and which have no hysteresis, thresholdless anti-ferroelectric liquid crystals (which is also called V-shaped switching liquid crystals and which will be hereinafter referred to as xe2x80x9cTLAF liquid crystalsxe2x80x9d), polymer stabilized ferroelectric liquid crystals (which will be hereinafter referred to as xe2x80x9cPS-FLC liquid crystalsxe2x80x9d), monostable FLCs showing V-shaped switching, and half V-shaped switching FLCs are widely noticed.
However, liquid crystal displays using liquid crystal materials having spontaneous polarization are characterized in that it is more difficult to realize a uniform alignment having no defect than twisted nematic (TN) liquid crystals, since such displays have a layer structure. Moreover, it was found that optical response characteristic, having no hysteresis, of TLAF, monostable FLCs showing V-shaped switching, and half V-shaped switching FLCs are induced by alignment layers. There are some cases where the TLAF phases are not provided by some kinds of alignment layers even if the same liquid crystal material is used. Therefore, in order to use a smectic liquid crystal with optical response characteristics having no hysteresis for a light modulating layer of a liquid crystal display, it is important to elect the optimum alignment layer to ensure sufficient alignment stability.
As a typical example, the TLAF liquid crystal will be described below. In an ideal TLAF liquid crystal, an average optical axis during the application of a voltage of 0 V (which will be hereinafter referred to as an optical axis from layer normal) is coincident with the normal direction of a smectic layer. When the TLAF liquid crystal is used as a light modulating layer of a display element, two polarizing plates, the polarizing axes of which are orthogonal to each other and which have the crossed Nicols configuration, are arranged in front and behind a liquid crystal panel, and the normal direction of the smectic layer is arranged so as to be coincident with one of the polarizing axes. In this case, there is obtained a transmittance-voltage characteristic shown in FIG. 11, wherein black is displayed when a voltage of 0 V is applied and a half tone to white is displayed when a positive or negative voltage is applied. However, in the case of some combinations of liquid crystal materials having a smaller spontaneous polarization than 120 nC/cm2 with alignment layers, there was observed a phenomenon that a domain, which had a stripe shape parallel to the direction of the smectic layer and in which the optical axis was deviated, was produced to grow with the elapse of time and/or that the deviation of the optical axis increased. In addition, in the case of some liquid crystal materials, the same domain in which the optical axis was deviated was produced when a voltage which was higher than or corresponding to a saturation voltage was applied. The deviated angle of the optical axis is narrower than the corn angle (xcex8). The reason for this is that the apparent tilt angle is narrow since the smectic layer has chevron structure, not bookshelf. This phenomenon that the optical axis is shifted from the normal direction of the layer will be hereinafter referred to as alignment deterioration.
Referring to FIGS. 12(a) through 12(d), examples of observed alignment deterioration will be described below. FIG. 12(a) is a schematic diagram of a smectic layer structure. Alignment layers provided on both substrates are rubbed at predetermined angles, and a TLAF liquid crystal material is introduced between the substrates, so that a smectic layer structure shown in the figure is formed. FIGS. 12(b) and 12(c) are schematic diagrams showing the alignment state when this panel is observed by a microscope wherein polarizing plates are arranged in the crossed Nicols configuration. Furthermore, if one of the polarizing directions of a polarizer and analyzer is shifted from the normal direction of a smectic layer by xxc2x0 as shown in FIG. 12(d), it is easy to observe the deteriorated domain.
In an alignment wherein no alignment deterioration occurs, the optical axis is one direction as shown in FIG. 12(b), so that light uniformly transmits to be visible. However, in a state that a domain having an optical axis deviated by xc2x1xxc2x0 is produced, i.e., in a deteriorated alignment, it is observed that a domain having an optical axis coincident with the polarizing direction is dark, and a domain having an optical axis deviated in the opposite direction is bright, in a uniform alignment as shown in FIG. 12(c). In the application to a display element, if it is not possible to completely inhibit this alignment deterioration, light leakage in a dark state is caused to deteriorate contrast.
As measures to inhibit deterioration, Japanese Patent Laid-Open No. 10-319377 has proposed a method for introducing a polymer precursor into a TLAF liquid crystal material, injecting them between substrates, and photopolymerizing them in SA phase to stabilize the structure when a voltage of 0 V is applied.
However, the inventors studied and verified that according to the method for introducing the polymer precursor as disclosed in Japanese Patent Laid-Open No. 10-319377, the alignment of the TLAF liquid crystal is disturbed by foreign molecules other than the liquid crystal material, to increase light leakage in a dark state regardless of polymerization methods, so that contrast lowers.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a liquid crystal display element having a good display performance, which is not influenced by the deterioration with the time course and the history of applied voltage.
In order to accomplish the aforementioned and other objects, according to a first aspect of the present invention, a liquid crystal display element comprises: a first electrode substrate having a first transparent substrate, a first electrode formed on the first substrate, and a first alignment layer formed on the first substrate; a second electrode substrate having a second transparent substrate, a second electrode formed on the second substrate, and a second alignment layer formed on the second substrate; and a light modulating layer of a smectic liquid crystal material which is sandwiched between the first and second electrode substrates and which has a thresholdless voltage-transmittance characteristic and a spontaneous polarization of 120 nC/cm2 or less, the polarity force component of the surface free energy of each of the first and second alignment layers being 13 dyn/cm or less.
Furthermore, the pretilt angle measured by a nematic liquid crystal with respect to the first and second alignment layers is preferably less than 6 degrees.
More preferably, the polarity force component of the surface free energy is 9 dyn/cm or less.
According to a second aspect of the present invention, a liquid crystal display element comprises: a first electrode substrate having a first transparent substrate, a first electrode formed on the first substrate, and a first alignment layer formed on the first substrate; a second electrode substrate having a second transparent substrate, a second electrode formed on the second substrate, and a second alignment layer formed on the second substrate; and a light modulating layer of a smectic liquid crystal material which is sandwiched between the first and second electrode substrates and which has a thresholdless voltage-transmittance characteristic and a spontaneous polarization of 120 nC/cm2 or less, the dispersion force component of the surface free energy of each of the first and second alignment layers being 38 dyn/cm or more.
Furthermore, the pretilt angle measured by a nematic liquid crystal with respect to the first and second alignment layers is preferably less than 6 degrees.
More preferably, the dispersion force component of the surface free energy is 42 dyn/cm or more.